Typical contemporary command-to-line-of-sight missile systems operate by optical measurement of the deviation of the missile from the desired line-of-sight and by commands to the missile to close the loop and restore the missile to this desired path. The optical tracker is located at the launcher. Deviation of the missile from the desired path results in correctional commands being encoded and coupled to a transmitter from which commands are transmitted via a data link which may be radio, optical, or wire. A beacon is employed on the missile to provide a source for the optical tracker. The beacon is modulated to provide a signature to permit the tracker to recognize the beacon and discriminate against background radiation. The contemporary systems employ amplitude modulation. The accuracy of the tracking, or error measurement, required in the missile system precludes the use of a mechanization which depends on the magnitude of the optical signal. In addition to the large changes in magnitude produced by the change in distance (range), variations in missile attitude and atomspheric phenomena produce significant changes in signal level. The classic method for error measurement, employed in most systems, utilizes beacon image nutation on a cruciform or quadrant detector, as shown in FIG. 1. The center of optical nutation of the image nutator 2 is aligned with the boresight axis of the sight which establishes the desired line-of-sight (LOS) and flight path. When the missile beacon image is on the LOS, the nutated image 4 describes a circle which is concentric with the center of the detector 6 as shown in FIG. 2. When the beacon is not on the LOS, the nutated image 4 is not concentric with the detector 6 center and appears as shown in FIGS. 1 and 3.
Measurement of beacon (and missile) LOS errors is made from the geometry and the times at which the image crosses the detector cross hairs. For example, the vertical error E.sub.v is equal to r sin .theta., and the horizontal error E.sub.h is equal to r sin .phi. where r equals nutation radius in feet, .theta. equals vertical nutation angle error in radians, and .phi. equals horizontal nutation angle error in radians. In actual practice, the nutation radius angle may be programmed as a function of time, based on missile range, to provide a constant value for r. The quantities sin .theta. and sin .phi. are obtained by sampling nutation reference generator signals and are the times of crossing the horizontal and vertical detector cross hairs. The accuracy of the error measurement is a direct function of the accuracy in determining the time of the crossings. A method of signal processing which has been used in the past is shown in FIG. 4. A detector 10 has four detector quadrant signals A1, B1, A2, and B2 which are processed into two channels. One channel 12 provides a reference signal whose amplitude is constant. The other channel 14 provides a signal of the same frequency, but which reverses phase 180 degrees from one quadrant to the next. A phase detector 16 is then employed to provide an output which changes level at the detector crossover points. The phase detector output is then used to initiate sampling of the nutation reference signals to provide error readings in vertical and horizontal channels.